feat: add new Hill cipher encryption algorithm implementation

[mmohamedkhaled]

Description of Change

This pull request adds an implementation of the Hill cipher encryption algorithm in C++.
The code uses modular matrix multiplication over a key matrix and processes input through cleaning and padding.
It also includes a test() function that uses assert() to verify output, and documentation follows Doxygen standards.

Checklist

• Added description of change
• Added file name matches File name guidelines
• Added tests and example, test must pass
• Added documentation so that the program is self-explanatory and educational - Doxygen guidelines
• Relevant documentation/comments is changed or added
• PR title follows semantic commit guidelines
• Search previous suggestions before making a new one, as yours may be a duplicate.
• I acknowledge that all my contributions will be made under the project's license.

Notes: Implements Hill cipher in ciphers/hill_cipher_2.cpp using standard C++ with Doxygen-compliant documentation and self-testing.